Synchronous receivers with direct conversion are receivers in which the received signal (antenna signal or amplified antenna signal) is demodulated by synchronous conversion into baseband. This conversion is carried out on two channels using two orthogonal oscillator signals (with a 90 degree phase difference). The oscillator signals are taken from a voltage-controlled oscillator with outputs for two signals with a 90 degree phase difference (two-phase oscillator). This voltage-controlled oscillator (VCO) is part of a phase-locked loop (PLL) by means of which the oscillator is made to follow the phase position and thus also the frequency of the received signal. During the phase lock condition, i.e., synchronous operation of the phase-locked loop, the amplitude-demodulated signal occurs as a result of the synchronous mixing at the output of the first mixer in the in-phase channel which is fed the oscillator signal (0 degrees) with no difference in phase from the input signal. Hence, this mixer operates as a synchronous amplitude demodulator. The phase-demodulated signal arises at the output of the second mixer in the quadrature channel which is fed the oscillator signal with a 90 degree phase difference from (in quadrature with) the received signal. This mixer is operated as a phase detector which supplies through a regulating filter the control signal for resetting the voltage-controlled oscillator. Envelope detection is carried out by addition of the squared output signals of the in-phase channel and the quadrature channel. The resulting positive, quadratic envelope signal is proportional to the power of the received signal at the mixer inputs and is required as a control signal for the automatic gain control (AGC) of the synchronous receiver. The envelope signal contains the (amplitude) modulation of the received signal.
Synchronous receivers with direct mixing have the disadvantage that undesired whistling and clicking noise occurs during receiver tuning. There is also the danger of missing the desired transmitter if the tuning is carried out too quickly. In contrast to receivers with demodulation by means of amplitude detection (superheterodyne receivers), the output signal of the synchronous demodulator does not necessarily become clearer as the tuning becomes more and more precise, but is only available at the instant of synchronization of the phase-locked loop. Prior to that, only an undesired whistling is received.